Aww, he looks like such a lovely little guy! Such a shame hes so clamped...Hopefully that does change after a while. :)

Since you just brought him home yesterday, he probably is just a little stressed from the transition and clamped from the horrible conditions in the pet store. But does he struggle at all to get to the top or does it look more like hes just hiding? What have you offered him to eat so far, and has he taken anything? What is your setup like for him?

He seems to swim fine for the most part. Has a little trouble maneuvering with his tail, given that it's so clamped up (but he can still reach the surface just fine).

Feeding him Aqueon betta bits at the moment. I'm looking to get some Omega One Betta Buffet pellets but the store was out of them. He did eat some last night (and he seemed super hungry). I think because he's always at the bottom of the tank he doesn't notice when it's feeding time.

Setup is a heated, filtered 5.5 gallon with some tetras and two cories. It worked wonderfully for my last betta, who sadly passed away Sunday morning due to a swim bladder issue.

I think it's normal for him to hide. He might just be shy and adjusting to his new home especially since he is sharing it with other fish...hiding might make him feel more secure untik he gets used to it. Hopefully the clamping will get better once he becomes more comfortable in his new home. Have you tried tempting him with some brine shrimp or bloodworms?

If he doesn't seem to have any maneuvering issues that don't seem to relate to his clamped tail, then hes likely just stressed and hanging out taking everything in. Its definitely a good sign that hes eating something, and if hes super hungry he'll probably get the idea of where the food comes from in a few days or so. ;)

Is your filter baffled at all? Could he flow possibly be bothering him?

Thats a lot of fish in a 5.5 though.....quite over stocked, both space and bio-load wise. Its possible the movement and presence of the other fish is stressing this guy out a little more even. Also, since the betta is in such bad shape, you never know what other things might be wrong with him....it may be better for everyone if you separate him out into a smaller, heated QT tank where you can better monitor his progress and treat him of anything else comes up without risking your other fish until hes healed up some.

However, since you are so over stocked(with the exception of dividing a tank for two bettas, in order to have actual fish tank mates you need a 10+ gallon setup), and taking into consideration that tetras and corys are both schooling/shoaling fish and should really be in groups of 5-6+, I would highly suggest looking into an upgrade/couple new setups for these other fish or considering rehoming options. To keep them together, each in proper groups, you'd need a 15-20+ gallon I'm afraid.

If the tank is small enough and acrylic, you could probably float/set it in the 5.5 to keep things nice and warm for him.

Temporarily you're OK so long as you keep up with water changes and watch your levels, but for the long-term health and well-being of your fish moving both species out and bumping up their numbers to 5-6 would be a good idea. Since you've got two spices that need schools/shoals of their own, see if you can upgrade to something a little larger or at the very least find another spare 10 gallon for the tetras.....you can usually find some nicely sized used tanks at thrift stores or on Creigslist if price is an issue.